The present invention relates generally to water reclamation from cooling tower effluent or other heat rejection devices. More particularly, the present invention relates to method and apparatus for reclaiming water from cooling tower effluent to provide a source of clean water, reduce water consumption of the cooling tower, and/or to reduce the cooling tower plume.
In electricity generation using steam driven turbines, water is heated by a burner to create steam which drives a turbine to creates electricity. In order to minimize the amount of clean water necessary for this process, the steam must be converted back into water, by removing heat, so that the water can be reused in the process. In air conditioning systems for large buildings, air inside the building is forced passed coils containing a cooled refrigerant gas thereby transferring heat from inside the building into the refrigerant gas. The warmed refrigerant is then piped outside the building where the excess heat must be removed from the refrigerant so that the refrigerant gas can be re-cooled and the cooling process continued.
In both of the foregoing processes, and numerous other processes that require the step of dissipating excess heat, cooling towers have been employed. In wet type cooling towers, water is pumped passed a condenser coil containing the heated steam, refrigerant, or other heated liquid or gas, thereby transferring heat into the water. The water is then pumped to the top of the cooling tower and sprayed over a cooling tower media comprised of thin sheets of material or splash bars. As the water flows down the cooling tower media, ambient air is forced passed the heated water and heat is transmitted from the water to the air by both sensible and evaporative heat transfer. The air is then forced out of the cooling tower and dissipated into the surrounding air.
Cooling towers are highly efficient and cost effective means of dissipating this excess heat and thus are widely used for this purpose. A recognized drawback to cooling towers, however, is that under certain atmospheric conditions a plume can be created by moisture from the heated water source evaporating into the air stream being carried out of the top of the cooling tower. Where the cooling tower is very large, as in the case of power plants, the plume can cause low lying fog in the vicinity of the cooling tower. The plume can also cause icing on roads in the vicinity of the cooling tower where colder temperatures cause the moisture in the plume to freeze.
Efforts have therefore been made to limit or eliminate the plume caused by cooling towers. Examples of such efforts can be found in the following United States Patents:
U.S. Pat. No. 6,247,682 to Vouche describes a plume abated cooling tower in which ambient air, in addition to being brought in at the bottom of the tower and forced upwards through a fill pack as hot water is sprayed down on the fill pack, is brought into the cooling tower through isolated heat conductive passageways below the hot water spray heads. These passageways which are made from a heat conductive material such as aluminum, copper, etc., allow the ambient air to absorb some of the heat without moisture being evaporated into the air. At the top of the tower the wet laden heated air and the dry heated air are mixed thereby reducing the plume.
U.S. Pat. No. 4,361,524 to Howlett describes a plume prevention system in which the hot water is partially cooled before being provided into the cooling tower. The partial cooling of the hot water is performed using a separate heat exchanger operating with a separate cooling medium such as air or water. As discussed in the patent, the separate heat exhanger reduces the efficiency of the cooling tower and thus should only be employed when atmospheric conditions exist in which a plume would be created by the cooling tower.
Another example of a system designed to reduce plume in a wet type cooling tower can be found in the xe2x80x9cTechnical Paper Number TP93-01xe2x80x9d of the Cooling Tower Institute 1993 Annual Meeting, xe2x80x9cPlume Abatement and Water Conservation with the Wet/Dry Cooling Tower,xe2x80x9d Paul A. Lindahl, Jr. et al. In the system described in this paper, hot water is first pumped through a dry air cooling section where air is forced across heat exchange fins connected to the flow. The water, which has been partially cooled, is then sprayed over a fill pack positioned below the dry air cooling section and air is forced through the fill pack to further cool the water. The wet air is then forced upwards within the tower and mixed with the heated dry air from the dry cooling process and forced out the top of the tower.
While the foregoing systems provide useful solutions to the wet cooling tower plume problem, they all require the construction of a complex and costly wet and dry air heat transfer mechanism. A simple and inexpensive wet and dry air cooling mechanism is still needed wherein dry heated air and wet laden heated air can be mixed before passing out of the cooling tower to thereby reduce the plume.
Another recognized problem with cooling towers is that the water used for cooling can become concentrated with contaminates. As water evaporates out of the cooling tower, additional water is added but it should be readily recognized that contaminants in the water will become more concentrated because they are not removed with the evaporate. If chemicals are added to the cooling water to treat the water these chemicals can become highly concentrated which may be undesirable if released into the environment. If seawater or waste water is used to replace the evaporated water, a common practice where fresh water is not available or costly, salts and solids in the water can also build up in the cooling water circuit As these contaminants become more concentrated they can become caked in between the thin evaporating sheets diminishing the towers cooling efficiency.
To prevent the foregoing problem it is a regular practice to xe2x80x9cblowdownxe2x80x9d a portion of the water with the concentrated contaminants and replace it with fresh water from the source. While this prevents the contaminants in the cooling tower water from becoming too concentrated, there may be environmental consequences to discharging water during the blowdown process. Efforts have therefore been made to reduce the water consumption in cooling towers.
U.S. Pat. No. 4,076,771 to Houx, et al. describes the current state-of-the-art in reducing the water consumption in a cooling tower. In the system described in this patent both cooling tower evaporative heat transfer media and a coil section that transfers heat sensibly are provided in the same system. The sensible heat transfer of the coils provides cooling of the process water but does not consume any water.
While the foregoing patent represents a significant advancement over prior art cooling towers, it would be desirable if a mechanism were developed for recapturing water from the plume for replacement back into the cooling tower water reservoir which did not require a coil section for sensible heat transfer.
A separate problem that has been noted is the desalination of sea water, and purification of other water supplies, to create potable drinking water. Numerous approaches have been developed to remove purified water from a moist air stream. The major commercial processes include Multi-Stage Flash Distillation, Multiple Effect Distillation, Vapor Compression Distillation, and Reverse Osmosis. See xe2x80x9cThe Desalting ABC""sxe2x80x9d, prepared by O.K. Buros for the International Desalination Association, modified and reproduced by Research Department Saline Water Conversion Corporation, 1990. Examples of systems that use low temperature water for desalination or waste heat include the following:
xe2x80x9cZero Discharge Desalinationxe2x80x9d, Lu et al, Proceedings from the ADA North American Biennial Conference and Exposition, August 2000. This paper provides information on a device that produces fresh water from a cold air stream and a warm moist air stream from a low grade waste heat source. The fresh water is condensed along the walls separating the two air streams. Also, a cold water is sprayed over the warm moist air to enhance condensation.
xe2x80x9cOpen Multiple Effect Desalination with Low Temperature Process Heatxe2x80x9d, Baumgartner et al, International Symposium on Desalination and Water Re-Use, Vol. 4, 1991. This paper provides information on a plastic tube heat exchanger used for desalination that uses cold running water on the inside of the plastic tubes and warm moist air flowing over the exterior of the tubes. The condensate forms on the outside of the cold tubes.
The foregoing show that there is a need for desalination systems for converting sea water, or other water supply containing high levels of contaminants, into a purer water supply. A simple and cost effective means of condensing the effluent of a cooling tower as a source of water would therefore be desirable.
In one aspect of the invention a heat exchanger is provided having a first set of passageways formed for receiving a first stream of air. A second set of passageways for receiving a second stream of air is also provided in the heat exchanger, the second stream of air being warmer than said first stream of air. Each passageway of the first set of passageways is separate but adjacent to at least one passageway of the second set of passageways so that heat from said second air stream will be absorbed by the first air stream. A reservoir for capturing moisture that condenses out of said second air stream is also provided.
In another aspect of the invention a heat exchanger is provided having two opposing walls configured with holes to allow for the passage of a first air stream. Tubes are provided between a hole in the first wall and a corresponding hole in the second wall for channeling the first air stream there through. Walls provided between at least two parallel edges of one wall and the corresponding parallel edges of said second wall ensure that a second air stream can be channeled passed said tubes to condensed moisture out of the second air stream. In another aspect of the invention a method of reducing the moisture content of an air stream is provided wherein a first air stream having a flow rate between 10 and 80 pounds of dry air per square foot per minute (pda/ft2/min) and a relative humidity at or above 90% is directed through a first set of passageways. A second air stream having a flow rate between 10 and 80 pda/ft2/min and a dry bulb temperature at least five Fahrenheit degrees below the second stream is directed through a second set of passageways. Each passageway of the first set of passageways being separated from at least one passageway of the second set of passageways by a thin heat conductive material. Heat from the second air stream is absorbed into the first air stream and water condensed out of the second air stream is captured. In yet another embodiment of the invention, a cooling tower is provided having a counterflow evaporative media and a water distribution system that distributes hot water over the counterflow evaporative media. A heat exchanger that absorbs heat from a first air stream into a second air stream is also provided, the heat exchanger having a first set of passageways and a second set of passageways. A fan in the cooling tower directs air through the counterflow evaporative media to create said first air stream and directs the first air stream, having a flow rate between 10 and 80 pounds of dry air per square foot per minute (pda/ft2/min) and a relative humidity at or above 90%, through the first set of passageways. The fan also directs the second air stream having a flow rate between 10 and 80 pda/ft2/min and a dry bulb temperature at least five Fahrenheit degrees below the second stream through the second set of passageways. Each passageway of the first set of passageways being separated from at least one passageway of the second set of passageways by a thin heat conductive material. A reservoir is provided for capturing water condensed out of the first air stream.
In another aspect of the invention a cooling tower is provided having a fan at the top of the cooling tower for creating a negative pressure inside the cooling tower. A counterflow evaporative media is provided along with spray heads that spray hot water onto the counterflow evaporative media. A heat exchanger having a first set of passageways for passing an air stream from outside the cooling tower into the center of the tower and a second set of passageways for passing an effluent air stream from the evaporative media is also provided in the heat exchanger. The air stream from outside the cooling tower absorbs heat from the effluent air stream and thereby condenses water out of the effluent.
In yet another aspect of the invention, a cooling tower is provided with a fan at the top of the cooling tower for creating a negative pressure inside the cooling tower. A crossflow evaporative media and a hot water distribution system that sprays hot water onto the crossflow evaporative media are provided. A heat exchanger having a first set of passageways for passing a first air stream from outside the cooling tower into the center of the tower and a second set of passageways for passing an effluent air stream from said evaporative media is provided. The air stream from outside the cooling tower absorbs heat from the effluent air stream and thereby condenses water out of the effluent.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.